Intraspecific cell size variability in the marine diatom Chaetoceros affinis along a nutrient gradient

The role of interspecific variability on the functioning of phytoplankton communities along environmental gradients has been well studied, while today still too little is known about the role of intraspecific variability, which may occur at an inter- and/or intragenotypic level. Nine different genotypes of Chaetoceros affinis were incubated under different nitrate concentrations: first altogether and with another stably coexisting species, the coccolitophore Emiliania huxleyi, through a long-term community experiment (199 days); then individually, through a short-term plasticity experiment (7 days). A significant intraspecific variability in cell size of Chaetoceros affinis was detected in both the experiments. According to a trait-based perspective, while no correspondence was observed among the cell size and nitrate concentrations at the intergenotypic level, the biggest cells were detected at the highest nitrate concentrations at the intragenotipic level in the community experiment in the long-term. This drove an increase in C. affinis mean cell size and total biomass along the nitrate gradient. In the plasticity experiment, significant differences in cell size along the nitrate gradient determined important variations in cellular nutrient content and stoichiometry at both the inter- and intragenotypic level. This work provides new findings on the intraspecific variability in phytoplankton morpho-functional traits and underlines the importance of considering this when phytoplankton responses to environmental changes are investigated.